The invention relates to the manufacture of elastomer-covered rollers.
Rollers with covers of elastomeric material are used in a wide variety of industrial applications. One process is a plastic film manufacturing process known as cast film extrusion. Films of thermoplastic polymers such as polyvinylchloride and polycarbonate are made by this process. The most popular polymers made by this process are polyethylene and polypropylene.
In a typical application, molten polymer from an extruder is dropped into a nip formed between a chrome-plated steel roller and an elastomer-covered steel roller. One common type of elastomer used in this application is silicone rubber, although Hypalon(trademark), available from Du Pont, and other polymer materials can also be used. The chrome-plated steel roller and the elastomer-covered roller are normally water cooled, or otherwise chilled, since the molten plastic may be at a temperature of several hundred degrees Fahrenheit. The chrome-plated roller quickly chills the plastic below its melting point and the nip formed with the elastomer-covered roller determines the gauge (thickness) and the surface finish of the film. The flexibility of the rubber covering compensates for minor machine misalignment and other variables, and provides a wider nip than two hard surfaced rollers.
In some applications, the film manufacturer also runs the surface of the silicone roller through a water bath to provide a measure of release (non-stick) from the tacky semi-molten plastic, and to provide additional cooling of the rubber surface. The excess water is removed with a doctor blade or other device and most of the remainder quickly evaporates from the hot roller surface. A small amount of the water (or water vapor) does pass through the extrusion nip. Because of the temperature difference between the rubber roller surface and the core/rubber bond line, water vapor tends to migrate through the rubber covering and condense at the core surface. Although the interface between the core and other materials is an area, this will be represented by the term xe2x80x9cbond linexe2x80x9d which is the portion seen in the drawings.
The vapor pressure of water at the hot roller surface may be several psi (pounds per square inch), while the vapor pressure at the core surface (typically 40 to 70xc2x0 F.) is a fraction of one psi. This pressure difference drives the water vapor through the wall of the silicone rubber covering. Silicone rubber, in particular, is quite permeable to water vapor. Other polymers that may be used for the rubber covering, such as Hypalon, are either less permeable to water vapor or are more hydrophilic and absorb the water. These polymers are still susceptible to water vapor migration. The rate of migration is slower, however.
As used herein, the terms xe2x80x9cpermeablexe2x80x9d and xe2x80x9cimpermeablexe2x80x9d shall mean permeable and impermeable to moisture or to DOP-type plasticizers. xe2x80x9cMoisturexe2x80x9d shall mean water or water vapor, or both. Moisture and DOP-type plasticizers shall be included in the term xe2x80x9cbond-degrading fluidsxe2x80x9d, and the term xe2x80x9cfluidsxe2x80x9d shall include liquids and vapors.
Even in applications where the rubber nip roller is not run through a water bath, migration of water vapor to the core is still known to be a problem, especially if the core temperature is below room temperature or the dew point. Rollers used for the application of coatings frequently have problems with water accumulation at the rubber/core bond line. As a result, rollers applying water-based coatings are known to have corrosion and bond failure at the core. The water vapor migration problem is not limited to cast extrusion nip rollers.
Silicone rubber is a material that may be used as the cover in the elastomer-covered roller described for the above application. Silicone rubber is, however, a material that is difficult to bond to a metal roller core on a consistent basis. It is also difficult to maintain the bond in applications where the roller is used at high temperature and high pressure. In some cases, the silicone rubber may peel cleanly off the metal core without leaving any residue of rubber, indicating a loss of bonding. Loss of bonding in one area requires that the elastomer-covered roller be replaced.
Under high pressure, typically 100 pli (pounds per linear inch) or more, there is a stress concentration at the rubber-to-metal bond line causing the silicone to tear away from the core due to shearing forces. The best currently available method to achieve a consistent bond is to grit blast the surface of the core to a high roughness, about 500 Ra, before applying the bonding agents or primers to the core. Bonding agents are applied in thin layers, typically less than 1 mil.
Grit blasting has certain limitations, however. It is difficult to achieve surface roughnesses greater than 500 Ra. Even maintaining 500 Ra generally requires the constant use of new grit and attention to the grit blasting process variables. The grit-blasted core surface has a high Ra but it is not a complex, high profile, surface like Velcro which not only has a high surface area, but also has a lot of xe2x80x9chooksxe2x80x9d. So called xe2x80x9chooksxe2x80x9d improve the bond strength by trapping material within small surface features.
If a rougher surface were available, for example, in the 500 to 2000 Ra range, not only would the surface area available for bonding be greatly increased, but the location of the rubber-to-metal bond line would be thicker and more diffused. This would in turn diffuse the shearing forces trying to tear the rubber from the core. Simply tooling a thread pattern into the core does not achieve this goal, because the direction of the thread is nearly parallel to the applied forces in the roller nip rather than perpendicular to them.
An improved bonding surface is needed for elastomers that are weakly or inconsistently bonded, especially if the covered rollers are exposed to high temperature or pressures, or high moisture conditions. The bonding surface must provide a high surface area and surface roughness and yet be easily and consistently produced. A very high surface roughness will also diffuse the stress at the bond line improving the longevity of the rubber to metal bond.
Polyurethane is another material that can be used for the elastomeric cover in the present invention. For bonding polyurethane to supporting layers, primary reliance has been placed on chemical bonding, to be assisted by mechanical bonding. As with silicone-based materials, the limit of surface roughness available with current methods of mechanical bonding is about 500 microinches Ra.
In addition, water vapor easily permeates through either a silicone rubber layer or a polyurethane layer, and corrosion may occur at the bond line due to the collection of water vapor there. Moisture will migrate into the outer roller cover if the roller is either chilled or exposed to water in the application. It would be beneficial to provide a bond coat or layer that is resistant to such corrosion, as well as one providing a stronger mechanical bond.
Another application of rollers is in the embossing and calendering of polyvinylchloride (PVC) film. The PVC film is made from a rigid PVC resin and is plasticized with oils to produce a softer material. PVC film is typically plasticized with DOP (dioctyl phthalate, a synthetic ester type oil) or chemically similar materials. These forming type operations require that the film be heated in a range of from 350xc2x0 F. to 400xc2x0 F. The film can lose DOP at these temperatures due to migration and evaporation.
In either embossing or calendering, the arrangement of rollers is similar to cast film extrusion with a hard surface roller forming a nip with a rubber-covered roller. Due to high temperatures, the rubber-covered roller is typically internally water cooled. The temperature differential through the rubber cover creates the same type of migration problem with DOP as is seen with water vapor in the cast film process. DOP migrates through the rubber covering to the core where it can degrade the bond between the core and the rubber cover. The hydraulic force of the oil tends to break the bond between the core and the cover layer of material. This is a second type of bond degradation, in addition to corrosion by moisture described above.
This can occur in rollers with silicone rubber covers, as well as rollers with a multi-layer configuration using EDPM rubber at the core. These materials may be permeated by DOP-type plasticizers.
Therefore, a general object of the invention is to improve bonding techniques and protect the bond layer against these adverse effects of its operating environment.
The invention concerns a method of making a roller that includes thermally spraying a bond coat to substantially cover a portion of the core on which an elastomeric layer is to be bonded and in which the bond coat provides a surface roughness to assure a strong mechanical bond. This bond coat has at least one coat that protects the bond line, either by allowing fluids to migrate away from the bond line, and/or by shielding the core from corrosion caused by the condensation of fluids near the surface of the core.
The invention also relates to a roller resulting from the utilizing this manufacturing method.
The bond coat comprises a thermally sprayed material selected from a group of materials consisting of metals, metal alloys, ceramics and cermets.
In the prior art, core surfaces were prepared by sanding or blasting the metal core prior to the application of chemical bonding agents. These methods do not always produce the desired level of mechanical bonding. Thermal spraying has been known in the manufacture of ceramic covered rollers, but has not heretofore been applied to bond elastomeric layers to a metal core.
The invention provides improved constructions of elastomer-covered rollers. These elastomers include silicone, EPDM (ethylene-propylene-diene-monomer), urethane elastomers and other synthetic or natural rubber elastomers. Although the invention is described in examples in which the elastomeric layer is the outer layer, it would also be possible to add layers outside the elastomeric layer.
The invention may be practiced in further aspects by providing a two-layer bond coat in which an outer layer is permeable to allow migration of water and water vapor, while the layer next to the core is impermeable to provide a corrosion barrier.
In another embodiment two layers of permeable material are applied to the core, with the inner layer being provided for better bonding to the core and the outer layer being provided for better bonding to the elastomeric layer.
Other objects and advantages of the invention, besides those discussed above, will be apparent to those of ordinary skill in the art from the description of the preferred embodiments which follow. In the description, reference is made to the accompanying drawings, which form a part hereof, and which illustrate examples of the invention. Such examples, however, are not exhaustive of the various embodiments of the invention, and therefore, reference is made to the claims which follow the description for determining the scope of the invention.